This section provides background information related to the present disclosure which is not necessarily prior art.
Head up displays (HUD) are known and are currently used in many military and commercial aircraft. HUDs are also finding application in automobiles. Conventionally, a HUD unit is mounted in the automobile to project an image in front of the driver. Adapting HUD units for use in automobiles necessitates making the units rugged, reliable, cost effective, and functional in a wide range of environmental and ambient lighting conditions.
One problem with HUDs in automobiles is maintaining satisfactory image illumination over a wide range of ambient lighting conditions. These ambient lighting conditions span from the extremely bright daylight lighting conditions of facing direct sunlight to the very dim nighttime lighting conditions of unlit rural highways. Daylight conditions generally require a bright or high intensity light source so that the projected image is visible in the corresponding bright ambient light conditions. Conversely, nighttime or low light conditions require a dim or low intensity light source to provide a projected image that is safe and comfortable in the corresponding dim ambient light conditions.
A HUD can use a light sensor to detect the ambient lighting conditions. Such a light sensor, however, can in some cases detect a sudden change in ambient light conditions that does not truly reflect the ambient lighting conditions. For example, as an automobile passes under a series of street lights, the light sensor can read each successive street light as a bright ambient daylight condition and each space between the street lights as a dim ambient nighttime condition. The resulting quick toggling between daylight and nighttime HUD light source intensities is not only annoying, but can be unsafe and detrimental to the reliability and service life of the HUD.